1. Field of the Invention
The present invention relates to immersion lithography tools for forming an image on a substrate, and more particularly, to the use of porous material for fluid removal in a gas curtain type immersion element.
2. Related Art
A typical lithography tool includes a radiation source, a projection optical system, and a substrate stage to support and move a substrate to be imaged. A radiation-sensitive material, such as resist, is coated onto the substrate surface prior to placement onto the substrate stage. During operation, radiation energy from the radiation source is used to project an image defined by an imaging element through the projection optical system onto the substrate. The projection optical system typically includes a number of lenses. The lens or optical element closest to the substrate is often referred to as the “last” or “final” optical element.
The projection area during an exposure is typically much smaller than the imaging surface of the substrate. The substrate therefore has to be moved relative to the projection optical system to pattern the entire surface. In the semiconductor industry, two types of lithography tools are commonly used. With so-called “step and repeat” tools, the entire image pattern is projected at once in a single exposure onto a target area of the substrate. After the exposure, the wafer is moved or “stepped” in the X and/or Y direction and a new target area is exposed. This step and repeat process is performed over and over until the entire substrate surface is exposed. With scanning type lithography tools, the target area is exposed in a continuous or “scanning” motion. The imaging element is moved in one direction, while the substrate is moved in either the same or the opposite direction during exposure. After each scan, the substrate is then moved in the X and/or Y direction to the next scan target area. This process is repeated until all the desired areas on the substrate have all been exposed.
It should be noted that lithography tools are typically used to image or pattern semiconductor wafers and flat panel displays. The term “substrate”, as used herein, is intended to generically mean any work piece that can be patterned, including, but not limited to, semiconductor wafers and flat panel displays.
Immersion lithography systems use a layer of fluid that fills a gap between the final optical element of the projection optical system and the substrate.
One known way to maintain the immersion fluid in the gap between the last optical element and the imaging surface of the substrate is by submerging both in a container filled with immersion fluid. See, for example U.S. Pat. No. 4,509,852, incorporated by reference herein in its entirety.
In another approach, which is a variation of the above-described submersion type tool, a confinement plate is used for submerging the substrate to be imaged in the immersion fluid. For more details on confinement plate type immersion lithography tools, see U.S. patent application Ser. No. 11/523,595, incorporated by reference herein in its entirety.
Another known way of maintaining the immersion fluid within the gap of a lithography tool is with the use of a nozzle that surrounds the last optical element immediately above the area to be exposed on the substrate. For more information on nozzle type immersion lithography tools, see U.S. Patent Publications 2006/0152697, 2007/0222967 and 2006/0087630, each incorporated herein by reference in its entirety.
Yet another known way of maintaining the immersion fluid in the gap where exposure of the substrate is to occur is with the use of a gas curtain. With a gas curtain design, an immersion element, typically with gas inlets and outlets, surrounds the last optical element of the projection optical system. The gas inlets are used to create a curtain of gas surrounding the exposure area, maintaining the fluid localized within the gap under the last optical element. The gas outlets are provided to remove the gas and any immersion fluid that may escape from the gap. For more information on gas curtain type immersion tools, see for example U.S. Patent publications 2005/0007569, 2006/0087630, 2006/0158627, and 2006/0038968, and/or European Patent Applications EP 1 477 856 A1 and EP 420 299 A2, each incorporated by reference herein in its entirety for all purposes.
The problem with the aforementioned gas curtain design is that immersion fluid may evaporate, causing cooling of the substrate. This is a significant problem because cooling may cause the substrate to contract, resulting in alignment, overlay and/or printing errors.
In a gas curtain type immersion lithography tool, prevention of evaporative cooling is therefore needed.